This invention relates to thin-film metal conductor systems and methods for the manufacture thereof.
Recent advances in microelectronics have created a demand for cost-effective, thin-film conductor systems for joining the components of a board mounted electrical circuit. Commonly used conductor systems comprise superposed layers of titanium, palladium, and gold.
For example, U.S. Pat. No. 4,385,966, issued May 31, 1983 to Keller et al. discloses a method for fabricating resistors and capacitors with electrical interconnections of Ti, Pd and Au. Ti, Pd, Au are deposited on a substrate material which bears electrical components, such as NiCr resistors. First, a Ti adhesion layer is deposited to provide a surface on which a subsequently deposited layer can bond. This layer is required since highly conductive metals, such as gold, do not adhere well to common substrate materials such as aluminum oxide wafers. A Pd layer is then deposited, followed by at least one Au layer. Next, a standard etching technique is used to form the metal layers into desired conductive paths and to uncover the surfaces of resistors and other components. The completed unit is then annealed at an appropriate temperature to increase physical strength.
Another system that employs a titanium adhesion layer is described by Morabito et al. in an article entitled "Material Characterization of Ti-Cu-Ni-Au (TPCA)--A New Low Cost Thin-Film Conductor System," IEEE Transactions on Parts, Hybrids, and Packaging, Vol. PHP-11, No. 4, December 1975.
When a circuit is formed by a process of the type described, the surfaces of components are exposed, with the conductive paths touching, but not covering, the components. Some components, particularly NiCr resistors, are subject to damage by abrasion and by contact with moisture. To protect against damage, it is current practice to coat the exposed components with a polymer material.
Although effective, the prior systems suffer from a number of disadvantages. The material costs of all such systems are relatively high since multiple layers of Au are typically required. Although other highly conductive materials are less expensive than gold, they are not practical for use directly in contact with titanium. Copper, for example, cannot be plated directly onto titanium due to diffusion problems.
Diffusion of titanium into the overlying conductive material is undesirable for several reasons. Primarily, it prevents effective adhesion of the conductor system to a substrate. Also, diffusion of the more resistant adhesion layer undesirably increases the electrical resistance of the conductor material. Whenever titanium is used as the adhesion layer, it is difficult to manufacture conductor systems on a commercial scale since the manufacturing process must be strictly controlled to minimize the diffusion of titanium into the superposed layers. Even slight deviations from the prescribed process can cause considerable diffusion.
Furthermore, in the formation of the prior multi-metal systems, an extra process step is required to physically remove the titanium adhesion layer from the board in regions that are intended to be nonconductive, specifically, from the resistors and other components that are applied before the titanium layer. A separate etching step is required to remove the titanium from the resistors and other component surfaces, since the overlying titanium would otherwise form a conductive path bypassing each component. Providing a polymer cover layer is a further added cost. It would be very beneficial to eliminate the titanium etching and polymer coating steps from the manufacturing process.
Thus, a need presently exists for conductor systems which are highly conductive, and yet are formed from low-cost materials using simple manufacturing processes.